In many applications it is necessary to identify a substrate, for example a semiconductor wafer, with the aid of a marking located on the substrate. Said marking is preferably read out in an automated process. It is theoretically possible to scan the entire area of the substrate and read out the information that is acquired. For that purpose, the acquired information must be processed using suitable image processing methods in order first to locate the marking and then to detect its content.
As a rule, however, the identification markings are very small compared to the size of the substrate. With the procedure explained above, a large volume of useless information would be obtained. The result would be a great deal of calculation effort, so that a comparatively large amount of time would be needed in order to identify a substrate.
Hitherto, for this reason, the substrate is first subjected, prior to reading the identification marking (whose position on the substrate is known per se), to what is referred to in the literature as “prealigning.” U.S. Pat. No. 5,870,488, for example, describes prealigning methods in which a substrate to be identified is first placed in a noncentered position on a turntable. By means of a subsequent rotation of the substrate, its eccentricity with respect to the turntable is determined; and then a positional correction of the substrate is performed by a displacement of the substrate, so that the substrate is subsequently centered on the turntable.
As set forth in U.S. Pat. No. 5,870,488, this procedure, which is known e.g. from U.S. Pat. No. 5,238,354, is very complex because of the two-stage alignment process required therein.
As an alternative thereto, it is proposed in U.S. Pat. No. 5,870,488 to sense the actual position of the substrate while it is being conveyed to the turntable, and to perform a correction immediately upon subsequent placement onto the turntable. The result of this is that the substrate is always placed concentrically with the turntable. The angular position of the substrate can be sensed by means of a rotation of the turntable, so that all the information needed to place the identification marking in the sensing region of an optical character recognition (OCR) camera can then be derived therefrom. Here again, very accurate positioning of the substrate on the turntable is therefore necessary before the actual identification operation. The identification method proposed in U.S. Pat. No. 5,870,488 thus requires a first measurement operation with a subsequent calculation step, followed by a very accurate placement of the substrate on the turntable, another measurement operation with a subsequent calculation step, and lastly a movement to the marking, which is then read out in known fashion.